The following terms and abbreviations are herewith defined, at least some of which are referred to within the following description of the prior art and the present invention. It should be noted that the reference to the “present invention” or “invention” used herein relates to exemplary embodiments and not necessarily to every embodiment encompassed by the appended claims.
ACK Acknowledge
BSC Base Station Controller
BSSAP Base Station Subsystem Application Part
eNodeB E-UTRAN NodeB
EPS Evolved Packet System
E-UTRAN Evolved-UTRAN
GERAN GSM/EDGE Radio Access Network
G-MSC Gateway MSC
GSM Global System for Mobile Communication
HSS Home Subscriber Server
HLR Home Location Register
IAM Initial Address Message
IMSI International Mobile Subscriber Identity
ID-HO Inter-Domain Handover
LTE Long Term Evolution
MAP Mobile Application Part
MSC Mobile Switching Center
MME Mobility Management Entity
PS Packet Switched
RNC Radio Network Controller
SABM Set Asynchronous Balanced Mode
SAE System Architecture Evolution
SGSN Serving GPRS Support Node
SRI Send Routing Information
TA Tracking Area
UE User Equipment
UMTS Universal Mobile Telecommunications System
UTRAN UMTS Terrestrial Radio Access Network
VLR Visitor Location Register
WCDMA Wideband Code-Division Multiple Access
Referring to FIG. 1 (PRIOR ART), there is a diagram of an exemplary mobile telecommunications network 100 illustrating a reference architecture associated with CS Fallback which is related to the present discussion. The exemplary mobile telecommunications network 100 includes a SAE/LTE network 100a and a GSM network 100b (or UMTS network) which are described in detail in 3GPP TS 23.002 v.8.2.0 dated December 2007 and 3GPP TS 23.401 v.8.0.0 dated December 2007 (the contents of these documents are incorporated by reference herein). As such, those skilled in the art are familiar with the architecture and functionality of this exemplary mobile packet telecommunications network 100. Thus, for clarity only components such as the UE, MME, SGSN and MSC server and interfaces such as the SGs which are relevant to the present discussion are discussed in detail herein while the other well known components or entities like the E-UTRAN, UTRAN, GERAN and the interfaces S1-MME, LTE-Uu, Um, Gb, Uu, S3, IuPS, A, and IuCS are not discussed within this document. The SGs is based on the Gs interface procedures and is the reference point located between the MME and the MSC server. The SGs reference point is used for mobility management and paging procedures between the EPS and CS domains which are discussed in detail below with respect to FIGS. 2-5 (PRIOR ART). Referring to FIG. 2, there is a diagram used to illustrate one example of how the SAE/LTE network 100a and the GSM network 100b may provide overlapping coverage in one location. In this example, UE1 is attached to the SAE/LTE network 100a in LTE cell e3 which belongs to Tracking Area 1, TA1, and is controlled by eNodeB1a which is connected to MME1. The GSM network 100b also provides GSM coverage for the UE1 in the same location (as shown by the dashed lines 200a and 200b). In this case, the GSM coverage is provided by GSM cell c3 which belongs to Location Area 1, LA1, and is controlled by the BSC1a which is connected to MSC1/VLR1 and SGSN1. In this particular example, there is almost a one-to-one relationship between the GSM network's LAs and the SAE/LTE network's TAs (e.g. LA1 and TA1 provide coverage in the same area). Alternatively, the SAE/LTE network 100a and a UMTS network may also provide overlapping coverage in one geographical location. In this alternative case, the GSM network's BSCs would be replaced by a UMTS network's RNCs. Referring to FIG. 3 (PRIOR ART), there is a signal flow diagram that shows an example of how UE1 would perform a SAE/LTE Attach to become attached for the PS domain and a CS Attach to become attached for the CS domain based on the situation shown in FIG. 2. The steps are as follows:
1. UE1 is located in LTE cell e3 and performs a SAE attach (see FIG. 2).
2. UE1 sends an attach message with a CS Fallback indicator to MME-1.
3. MME-1 initiates the SAE attach per 3GPP TS 23.401 which involves UE1, eNodeB1a, MME1 and HSS.
4. MME1 decides that the UE1 is to be CS attached in MSC1/VLR1.
5. MME1 sends a location update message to MSC1/VLR1.
6. MSC1/VLR1 and HSS perform a normal location update.
7. MSC1/VLR1 sends a location update accept message to MME1.
8. MME1 sends an attach accept message to UE1.
Referring to FIG. 4 (PRIOR ART), there is a signal flow diagram that shows an example of how the CS Fallback can be performed for a MT call in the case where there is no LA/TA mismatch problem. The steps are as follows:
1. UE1 is SAE/LTE attached and CS attached in MSC1/VLR1 as shown in FIG. 3.
2. UE1 is still located in LTE cell e3.
3. G-MSC receives an IAM (MT call request).
4. G-MSC initiates a normal SRI procedure with MSC1/VLR1.
5. G-MSC sends the IAM to MSC1/VLR1.
6. MSC1/VLR1 sends a page message (with suitable LAs) to MME1.
7. MME1 sends the page message (with suitable LAs) to eNodeB1a which then interfaces with UE1.
8. eNodeB1a finds out that the best GSM cell belonging to the suitable LAs (associated with the page message) based on the current location of UE1 is c3. For instance, the eNodeB1a could make this determination based on measurement reports received from UE1 i.e. indicating how well UE1 “hears” the GSM cells. Another option could be to configure the eNodeB1a such that it knows an E-UTRAN/LTE cell is totally covered by a specific GSM/WCDMA cell.9. eNodeB1a triggers an Inter-Domain Handover procedure towards the GSM cell c3 to initiate the CS Fallback procedure.10. eNodeB1a sends a relocation request message (with target cell=c3) to MME1.11. MME1 sends the forward relocation request message (with target cell=c3) to SGSN1.12. SGSN1 sends a PS handover request message (with target cell=c3) to BSC1a. 13. BSC1a allocates PS domain resources and CS domain resources in target cell c3.14. BSC1a sends a PS handover request acknowledge message (“ID HO command message”) to SGSN1.15. SGSN1 sends a forward relocation response message to MME1.16. MME1 sends a relocation command to eNodeB1a. 17. eNodeB1a sends a handover from eUTRAN command to UE1.18. UE1 initiates GERAN A/Gb Access Procedures with BSC1a using the allocated PS domain resources.19. UE1 sends a SABM (paging response message) to BSC1a using the allocated CS domain resources.20. BSC1a forwards the paging response message to MSC1/VLR1. Note: steps 10-20 are a normal ID HO.21. MSC1/VLR1 initiates the MT call setup with UE1.
FIG. 5 (PRIOR ART) is a diagram used to help describe the LA/TA mismatch problem related to CS Fallback for Mobile Terminated calls. In this case, the GSM LAs and the SAE/LTE TAs are not fully coordinated (i.e. there is no 1-to-1 relationship between the cells). Two different locations for different points in time are shown for UE1. The “UE1-attach” indicates the location at the time when the UE1 performed the SAE/LTE Attach in LTE cell e3 and became also CS Attached to MSC1/VLR1 based on the sequence and principles shown in FIG. 3. The “UE1-later” shows the location of the UE1 later when it has moved (while in LTE Idle mode) to the LTE cell e1. As LTE cells e3, e2 and e1 belong to the same TA1, after and during moving to LTE cell e1, UE1 does not perform any mobility related signaling towards the SAE/LTE network 100a and the UE1 therefore remains CS Attached in MSC1/VLR1 (i.e. it does not become CS attached in MSC2/VLR2). The LA/TA mismatch becomes a problem in this particular case. If there is a MT call arriving at MSC1/VLR1 (as shown in step 5 of FIG. 4) for UE1 at the “UE-later” location, then there is no GSM cell in the current location of UE1 that is connected to MSC1/VLR1. Instead, GSM Cell c31 is connected to MSC2/VLR2. This means that the CS Fallback procedure in FIG. 4 is not sufficient in this case. In particular, if the CS Fallback procedure where to be performed as shown in FIG. 4, then UE1 would experience ID HO to GSM Cell c31 and the paging response message would be sent to BSC2 and MSC2/VLR2. But since MSC1/VLR1 holds the MT call request, there is no way for the MT call to succeed in this situation because MSC2/VLR2 will receive the paging response message and has no way of knowing how to relay the paging response message to MSC1/VLR1 (i.e. there is a mismatch between the MSC receiving the MT call request and the MSC receiving the paging response message). Thus, there has been a need to address this problem and other problems which are associated with the existing CS Fallback procedures. This particular need and other needs have been addressed by the present invention.